1. Field of the Invention
The present invention relates to an electronic flash apparatus utilizing, as a switching device for controlling start and termination of flash emission of a flash discharge tube, a voltage-controlled switching device such as an insulated gate bipolar transistor (IGBT).
2. Related Background Art
In the conventional electronic flash apparatus, a thyristor is usually connected serially with the flash discharge tube. However, in the use of such thyristor, there is required a known current diverting circuit for terminating the flash emission of the flash discharge tube, giving rise to drawbacks of an increased cost and an increase space required for current diverting circuit.
For avoiding such drawbacks, it has been proposed to replace the thyristor with a gate turn-off switching device as disclosed in the Japanese Pat. Publication No. Sho49-39416, or with a large-current bipolar transistor as disclosed in the Japanese Laid-open Pat. Nos. Sho58-197694 or Sho58-197695. However these devices are not employed in practice, since these devices are bulky and difficult to incorporate. Also the,-Japanese, Laid-open Pat. Nos. Sho61-50125 and Sho61-50126 propose the use of a large field effect transistor (FET), which is a voltage-controlled device, for controlling the flash emission current, but such device is not employed in practice due to a large loss in the FET.
The recently developed insulated gate bipolar transistor (IGBT) has started to be utilized as the light emission controlling switching device (hereinafter called flash emission control device) of the electronic flash apparatus. The IGBT is a voltage-controlled three-terminal switching device having a gate, a collector and an emitter, in which the conduction between the collector and the emitter can be controlled by a voltage applied between the gate and the emitter, and is characterized by a low loss in contrast to an FET.
The IGBT can be rendered conductive usually by applying a voltage of 20-40 V to the gate (control terminal) while the emitter is maintained at the ground potential, and rendered nonconductive by maintaining the gate and the emitter at a same potential. The power supply voltage (3 to 12 V in ordinary electronic flash units) is too low on-off control of the IGBT, but the voltage of the main capacitor for charge accumulation for the flash discharge tube (usually 200-500 V) is too high for the drive voltage for supply to the control terminal for on-off control of the IGBT. For this reason there is required a separate power source for controlling the IGBT, thus giving rise to the drawbacks of increased cost and therefor.
The Japanese Utility Model Publication Sho57-29520 proposes to facilitate the triggering of flash emission in the conventional electronic flash apparatus, by applying a voltage of about twice of that the main capacitor, between the anode and cathode of the flash discharge tube. The apparatus employs a thyristor as the flash emission control device, and the doubled voltage is obtained by applying the negative potential of the main capacitor to the cathode of the flash discharge tube.
It is conceivable to secure the necessary voltage by forming an intermediate tap in the secondary coil of the transformer of the DC-DC converter for charging the main capacitor, as in the apparatus employing an FET as the flash control device as disclosed in the Japanese Laid-open Pat. No. Sho61-50125 or Sho61-50126, or the apparatus employing a bipolar transistor as the flash control device as disclosed in the-Japanese Laid-open Pat. No. Sho58-197695 or Sho61-50125. However, since the voltage from the intermediate tap fluctuates when the voltage of the main capacitor constituting the load of the DC-DC converter drops immediately after the flash emission, it may become impossible to obtain the necessary voltage if the next flash emission is needed immediately. Also, in a flash apparatus in which the function of the DC-DC converter is stopped after the main capacitor is charged to a predetermined voltage, thereby eliminating the idling current of the DC-DC converter for energy economization, the necessary voltage cannot be obtained from the intermediate tap of the secondary coil when the function of the DC-DC converter is stopped.
Such drawback exists also in a structure, disclosed in the Japanese Laid-open Pat. No. Sho63-129327, FIG. 4, in which a coil is added to the transformer of the DC-DC converter.
It is also conceivable to activate the DC-DC converter in response to the flash start instruction, but the start of flash emission is delayed because the DC-DC converter has a relatively low oscillating frequency at the start of oscillation, thus requiring time for providing a sufficiently high voltage. Consequently, in case of synchronization with a focal plane shutter of a high shutter speed such as 1/250 sec., there may result an uneven exposure because the trailing shutter curtain starts to run before the termination of flash emission due to the above-mentioned delay.
When the IGBT is employed as the flash control device, the double method voltage method disclosed in the Japanese Utility Model Publication Sho57-29520 cannot be utilized as it cannot apply the negative potential to the collector of the IGBT, so that the IGBT is inferior in flash triggering to the thyristor.